I think WillaGwen is fine. Where did you get this feedback from? From people you know or on Nameberry?

I find that people can get hung up on what they feel is a "heavy" use of the same letter a lot on here, of course everybody has their opinions and Nameberry is great for getting a general opinion - but everybody on here has SUCH different tastes, so you can't let the feedback completely put you off a name that you love - particularly if you are using the name for sentimental reasons, it has SO much more meaning to you than people can understand so of course you'll feel differently about it than some random stranger on an internet forum will.

Personally I much prefer the name Willa to Wilhelmina, and I much prefer Gwen to Gwendolen - if WillaGwen is the name you love, absolutely use it. I, for one, think its pretty and a nice way to honour your relatives.